The objectives of the proposed research are to further define the process whereby skeletal muscle adapts to high-intensity (weight-lifting) exercise and to elucidate the mechanisms which control this process. Adult cats are operantly conditioned to flex their right wrist against increasing resistance to receive a food reward. This procedure has the advantage of inducing significant hypertrophy in the muscles of one limb, while the muscles of the opposite limb can be used for comparative studies. This exercise regimen has been shown to increase the number of fibers in the muscles of the exercised limb. The mechanism for this increase has been shown to be fiber splitting. In the proposed study, the structural and histochemical features of muscle fibers that are undergoing division by splitting will be characterized, and the adaptive changes that occur in the exercising muscle that contribute to fiber splitting will be investigated. Alterations in the contractile properties of the exercised muscle will be investigated, and pharmacological blocking drugs (d-tubocurarine and decamethonium) will be used to evaluate the component parts of mixed muscles. EMG will be used to assess adaptive changes in the function and recruitment of exercising muscles. The proposed study will further elucidate the dramatic structural and functional alterations that occur in skeletal muscle that is adapting to high-intensity exercise, and define the mechanisms that control these adaptive processes.